1. Field of the Invention
Embodiments of the invention relate generally to a thin film transistor having stable electrical properties, and methods of making such a transistor.
2. Description of the Related Art
Thin film transistors (TFT's) are widely used to make flat panel displays of many sizes and types. In general, a thin film transistor is formed in layers on a substrate. A conductive bottom gate layer is covered by a dielectric material to support maintenance of an electric field between the conductive bottom gate layer and a top gate layer to be formed later. A semiconductive layer is generally formed over the dielectric layer. The semiconductive layer acts as a supplier of electrons to the transistor channel, which is a doped semiconductive material formed over the active layer. The top gate contact is formed over the channel layer.
In operation, a gate voltage is applied to the gate and a bias voltage is applied to the channel through source and drain junctions. The gate voltage produces an electric field through the transistor by virtue of the dielectric layer. The electric field encourages electrons to move from the active layer into the channel layer. When enough electrons have migrated, a current flows through the channel layer.
To ensure reliable operation of the TFT, mobility of electrons in the active layer is important. Electrons must be free to migrate from the active layer into the channel layer readily in response to an applied gate voltage. If electron mobility in the active layer declines, the gate voltage required to generate a current in the channel increases, potentially causing failure of the transistor. In addition, stability of properties such as threshold voltage under thermal and electrical stress is key to reliable operation.
Thus, there is a continuing need for a thin-film transistor with stable properties and high electron mobility.